This calculator computes the heat index based on temperature and relative humidity using a formula developed by the National Weather Service.
The program is operated by entering a temperature in degrees Fahrenheit and a relative humidity in percent, and then pressing the Calculate button. All entries are cleared by pressing the Clear button. If either entry is out of range, a popup warning message will be displayed.
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The Java Script source code for this program can be viewed by using the View|Source command of your web browser.
You may use or modify this source code in any way you find useful, provided that you agree that the author has no warranty, obligations or liability. You must determine the suitablility of this source code for your use.
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The National Weather Service devised the "Heat Index" (HI) which is the "apparent temperature" for a hot humid day. It is given in degrees Fahrenheit and is an accurate measure of how hot it really feels. Since HI values were devised for shady, light wind conditions, exposure to full sunshine can increase HI values by up to 15°F. The HI temperature is calculated from the following formula:
THI =
16.923 + 1.85212·10-1·T + 5.37941·RH - 1.00254·10-1·T·RH + 9.41695·10-3·T2 + 7.28898·10-3·RH2 + 3.45372·10-4·T2·RH - 8.14971·10-4·T·RH2 + 1.02102·10-5·T2·RH2 - 3.8646·10-5·T3 + 2.91583·10-5·RH3 + 1.42721·10-6·T3·RH + 1.97483·10-7·T·RH3 - 2.18429·10-8·T3·RH2 + 8.43296·10-10·T2·RH3 - 4.81975·10-11·T3·RH3
In which,
T = temperature in degrees Fahrenheit [°F]
RH = relative humidity in percent [%]
The effects of high heat index values are given in the following table provided by the NWS:
| Category | Classification | HI [°F] | General Affect on People at High Risk |
|---|---|---|---|
| I | Extremely Hot | 130°F or Higher | Heat or Sunstroke highly likely with continued exposure |
| II | Very Hot | 105°F - 130°F | Sunstroke, heat cramps, or heat exhaustion likely, and heatstroke possible with prolonged exposure and/or physical activity |
| III | Hot | 90°F - 105°F | Sunstroke, heat cramps, or heat exhaustion possible with prolonged exposure and/or physical activity |
| IV | Very Warm | 80°F - 90°F | Fatigue possible with prolonged exposure and/or physical activity |
Source: Heat Wave/Heat Index
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